Olefins are traditionally produced from petroleum feedstock by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefin(s) such as ethylene and/or propylene from a variety of hydro-carbon feedstock. It has been known for some time that oxygenates, especially alcohols e.g. methanol, are convertible into light olefin(s). The preferred methanol conversion process is generally referred to as methanol-to-olefin(s) (MTO) process, where methanol is converted to primarily ethylene and propylene in the presence of a molecular sieve.
Some of the most useful molecular sieves for converting methanol to olefin(s) are the metalloaluminophosphates such as the silicoaluminophosphates (SAPO's). There are a wide variety of SAPO molecular sieves known in the art; of these the more important examples include SAPO-5, SAPO-11, SAPO-18, SAPO-34, SAPO-35, SAPO-41 and SAPO-56. For the MTO process SAPO molecular sieves having the CHA framework type (Atlas of Zeolite Framework Types, 2001, 5th Edition, p. 96) and especially SAPO-34 are particularly important catalysts. The CHA framework type has a double six-ring structure in an ABC stacking arrangement. The pore openings of the structure are defined by eight member rings that have a diameter of about 4.0 Å, and cylindrical cages within the structure of approximately 10×6.7 Å. Other SAPO molecular sieves of CHA framework type include SAPO-44, SAPO-47 and ZYT-6.
The synthesis of AlPO4 and SAPO molecular sieves is a complicated process. There are a number of variables, which need to be controlled in order to optimise the synthesis in terms of the purity, yield, and quality of the SAPO molecular sieve produced.
It is known that silicoaluminophosphates of relatively small particle size are particularly effective in the MTO process. De Chen, et al., reported that SAPO-34 crystals of 0.4 to 0.5 μm gave the largest capacity of olefin formation (Microporous and Mesoporous Materials, 29, 191-203, 1999). In this work, the crystals were obtained from a single batch of crystals, which was fractionated to obtain the differently sized crystals evaluated.
In U.S. Pat. No. 4,440,871, the synthesis of SAPO materials of various framework types is described. The reference suggests that, whilst not essential, seeding of the reaction mixture with seed crystals of either the SAPO species to be produced or a topologically similar aluminophosphate or aluminosilicate composition, facilitates the crystallization procedure. In Examples 51 and 53, SAPO-31 is prepared from a mono-templated reaction mixture in which di-n-propylamine is used as template in combination with AlPO4-31 seeds.
In WO 00/06493, colloidal crystalline molecular sieve seeds are used in the manufacture of phosphorus-containing molecular sieves. The use of these seeds produces phosphorus-containing molecular sieves of controlled final particle size of at most 0.75 μm and narrow particle size distribution. In the specific examples a dual template system of di-n-propylamine (DPA) and tetraethylammonium hydroxide (TEAOH) is used at a ratio of template to Al2O3 of 2.6:1, i.e., a molar template to aluminum ratio of 1.3:1.
In EP 0 541 915 A1, it was reported that metal aluminophosphate molecular sieves of reduced particle size may be manufactured by the use of high speed stirring of the reaction mixture when TEAOH is used as templating agent.
He, Changquing et al., also reported that the variation of the mole ratio of TEAOH:NEt3 in a dual templated synthesis of SAPO-34 allowed control of crystal dimensions (Journal of Molecular Catalysis (China), 8:3, 207-212 (1994); Chinese Journal of Catalysis (CUIHUA XUEBAO), 16:1, 33-37 (1995); Chinese Patent Application No. 1106715A). He, Changquing et al., also reported that variation in the composition of template enabled adjustment of the acid center distribution of the synthesized SAPO-34 (Journal of Fuel Chemistry and Technology, 23:3, 306-311 (1995)).
We have now found new methods for the manufacture of crystalline aluminophosphate or silicoaluminophosphate molecular sieves that allow control of the crystal size of the molecular sieve. In addition, the molecular sieve crystals obtained by these methods possess excellent catalytic properties, especially when used in catalytic processes for the preparation of olefins from oxygenate feed-stocks. These new methods also allow preparation of silicoaluminophosphate molecular sieves of the CHA framework type with low acid site density.